What can cause space maintainer (dental) pain?

Space maintainer. A space maintainer is used to keep the space open on a child's teeth where he loses a molar before it is time. It prevents tooth crowding, but once the permanent tooth starts to brake through the gum, the usefulness of the space maintainer is over and should be removed. The erupting permanent tooth could be the source of pain.
Anything new. Anything new takes a period of adjustment. Sometimes there are strains in the materials, sometimes it just takes moving parts (tongue, cheeks, etc, time to adjust. Level of discomfort should not be much more than annoying, should disappear in a very few days, and controllable w otc analgesics. If anything more intense than this, call your dentist.
May be time to remov. A space maintainer is used to keep the space open when a baby tooth is lost too soon. When a baby tooth is lost, the adjacent teeth may move into the space creating an alignment issue. When the secondary tooth is ready to come in the space maintainer may be in the way and cause pain. Consult with your dentist to see if it is time for removal. It may also be a hygiene issue.
Inflammation? Depending on timing (i.e.: how long you've had the space-maintainer), you could have pain from different areas. If it's new, there is likely some pressure & inflammation on teeth adjusting to it. If it's been there a while, it could be related to areas that are difficult to clean or pressure from the way your teeth function when biting. I recommend your provider look at it to ensure it's working.
Malfunction. A space maintainer that has previously not hurt, should not start hurting unless there is a problem. See dentist who placed space maintainer for assessment.